In recent years, polymerizable resin materials have been used as polymerizable compositions for dental use, widely in the entire fields of dental treatment, owing to the great processability such that the materials can appropriately be molded into desired shapes, the good aesthetic beauty such that the materials can restore damaged tooth as if the resulting tooth were natural, and the high strength of the cured product.
Since U.S. Pat. No. 3,066,112 issued in 1959 proposes the application of polyfunctional acrylic polymerizable resins to the treatment of caries, a great number of materials chemically polymerizable at ambient temperature have been put to practical use. Thereafter, a dental material of ultraviolet polymerization has been proposed by using benzoin methyl ether and similar catalysts, in parallel to the progress of materials of chemical polymerization (Japanese Patent Application Laid-Open No. 47-247).
Furthermore, a number of products have been developed, wherein a variety of polymerization processes have been applied, including a dental material of photo-polymerization using camphorquinone (Japanese Patent Application Laid-Open No. 48-49875).
Since then, photo-polymerizable materials containing camphorquinone have widely been used as filling materials for dental use.
For applications where no effective level of light is incident even after photo-irradiation, for example for bonding of metal crowns, inlay and onlay, furthermore, dental materials of chemical polymerization are currently used.
Additionally, Japanese Patent Application Laid-Open No. 60-32810 discloses a dental composition containing a catalyst system comprising one of .alpha.-diketone compounds, namely camphorquinone, an organic peroxide and an amine, and describes that the thickness of the resulting unpolymerized layer on the surface is small.
Further, Japanese Patent Application Laid-Open No. 60-89407 describes that a dental composition containing a catalyst system of the same composition as described above can attain very large curing depth.
These compositions for dental use, which contain a catalyst system comprising camphorquinone, organic peroxides and amines, have both the functions of chemical polymerization and photo-polymerization. Such composition shave been used widely as adhesives and bonding agents for dental use, since the compositions have got good appraisal such that the compositions can readily promote high polymerization degree on the surfaces of materials owing to the function of photo-polymerization and can also progress chemical polymerization even in depth where no sufficient light required for polymerization can reach.
However, camphorquinone itself has a color in vivid yellow, polymerizable compositions containing camphorquinone are colored yellow inappropriate for dental materials. Additionally, the vivid yellow of the polymerizable compositions containing camphorquinone fades through photo-irradiation, so that the color changes markedly between prior to and after polymerization. In recent years, the demand for aesthetic beauty from dental patients has been emphasized in the field of dental treatment, and it is said that the vivid yellow color and the color change between prior to and after polymerization are not suitable.
A combination of benzoyl peroxide, i.e. a polymerization initiator, and reducing agents such as tertiary amine, which has been used widely for chemically polymerizable compositions, is problematic in view of color stability in that the color of the cured products of such compositions readily changes when the products are exposed to hot water.
For the purpose of overcoming the problems, a dental composition containing acylphosphine oxide as a photo-polymerization initiator has been proposed (Japanese Patent Publication No. 6-55654). The cured products resulted from the photo-polymerizable compositions containing acylphosphine oxide as a photo-polymerization initiator are excellent in its color stability, but the photo-polymerizable compositions are disadvantageous in that polymerization is not sufficiently facilitated in depth because the curing depth of the composition is shallower than that of the polymerizable compositions containing camphorquinone.
A technique for improving the affinity between teeth and such cured products thereby increasing the adhesion strength between teeth and the cured products, by using an acidic monomer having an acid group together with a polymerizable unsaturated group for a part of polymerizable monomers to be used in dental materials, has been on the way of practical use. Though polymerizable compositions containing acidic monomers have such an advantage as stated above, the thickness of the resulting unpolymerized layer is so large that the hardness of the resulting cured product may be insufficient.
Because the burden over patients is less as the therapeutic treatment time is shorter, a dental material with such a high polymerization rate that can be cured for a short period has been expected.
Therefore, it is an object of the present invention to provide a polymerizable composition for dental use, the composition being colored suitable as a dental material, having less color change between prior to and after polymerization (prior to and after curing) and such a larger curing depth to produce cured products with sufficient hardness and strength even in depth.
Furthermore, it is the other object of the present invention to provide a polymerizable composition for dental use, in addition to the properties described above, being capable of producing cured products with such a small thickness of the unpolymerized layer on the surface and a higher hardness, having a larger polymerization rate so the composition can be cured for a short period of time, and having great adhesion to the teeth.